The present invention relates to a measuring device for measuring the rheological properties of a substance, said measuring device comprising a sensor to be arranged in the process, a drive and a shaft for rotating the sensor in the process, and a measuring element for measuring the torque produced between the sensor rotating in the process and the process.
Viscosity, consistency etc. can be mentioned as examples of rheological properties. It is thus necessary to perform measurements of this type in many different fields of technology. The above-mentioned measuring devices are thus largely known in many different fields. In connection with the manufacturing processes of the pulp industry, for instance, it is necessary to perform accurate measurements and to continuously control the consistency of pulp, i.e. the ratio of a solid substance such as groundwood pulp to the liquor it has sludged into. The basic principle is the measurement of the torque produced between a sensor rotating in the process and the process.
As additional examples of fields in which the above-mentioned measuring devices are used, the fields of technology relating to the processing of pulps containing cellulose and textile fibers and of different sludges can be mentioned.
As an example of the known solutions, the measuring device disclosed in Finnish Patent 40,754 can be mentioned. The technology on which the invention is based has been described in said publication.
A disadvantage of the above-mentioned known measuring device is for instance that the variations of the flow direction and speed of the pulp to be measured affect the measurement result. In addition, a problem is presented by the asymmetry of the solution, i.e. as the direction ore the flow changes, the flow does not reach one side of the slab sensor, wherefore there is no pulp flow on said side. Another disadvantage of the solution is presented by difficulties in mounting, because in practice, special connections have to used in connection with said solution. Yet another problem is caused by fouling, since the asymmetrical structure causes shadow areas to be produced, impurities thus accumulating in these areas. Correspondingly, it should be noticed that the speed of a slab sensor is lower in the middle section than in the edge area, impurities thus accumulating in the middle areas of the sensor. To arrange the cleaning of a sensor is relatively difficult to implement if the sensor is of a plate-like structure. A disadvantage is also presented by the fact that a slab sensor is subjected to great axial forces and bending forces due to the effect of the variation in the flow direction of the pulp, these forces possibly disturbing the reception of the measurement signal. Previously, problems have also been caused by phenomena which cause zero drift, i.e. hysteresis in connection with the leading of a measuring shaft caused by the process state; flow and pressure shocks caused by the process on the sensor, and shocks caused by the pieces propagating in the process on the sensor; mechanical changes caused by variations in temperature; the drift due to the aging of the measuring element; and the error torque due to the asymmetrical flow profile.